Generally, free-radical initiated polymerizations are highly exothermic reactions and although bulk, solution, emulsion and dispersion processes are possible, the latter three are used most often. This results from the moderating effect of the diluent in controlling the exotherm by reducing the viscosity of the medium thus enabling the reaction medium to be stirred easily, dissipating the excess heat of reaction.
While bulk copolymerization of vinyl esters is theoretically possible, the problems of heat dissipation have heatherto prevented the use of this process on a technical or commerical scale. The problem is essentially a cyclic one. As the polymerization proceeds it both produces heat and becomes more viscous. It must be efficiently stirred to dissipate the heat but to do so requires a higher temperature to reduce the viscosity and this in turn speeds up the reaction leading to a runaway polymerization. The larger the batch size, the more this problem is compounded.
On the other hand, a technically feasible bulk copolymerization process for vinyl esters and .alpha.-alkenyl aromatic hydrocarbons is very desirable. Polymers made from vinyl esters and styrene together with other unsaturated monomers containing cross-linkable functional groups like acids, amides, alcohols, esters and anhydrides such as those reported in U.S. Pat. No. 3,669,942 issued June 13, 1972 and U.K. Pat. No. 1,249,390 issued Oct 13, 1971 are useful in paints and coatings. These polymers are useful as weather resistant powder coatings, nonaqueous dispersion coatings, water-soluble paint binders and binders for solution paints. Producing these polymers by a bulk process offers the advantages that they can be used directly in solvent free coating formulations. Bulk polymerization increases the yield of copolymer per volume of reactor and avoids the drawbacks associated with removing solvents such as, energy consumption, extra equipment, and time consumption.
We have now found a novel bulk process for producing polymers containing vinyl esters and .alpha.-alkenyl aromatic hydrocarbons which comprises the gradual addition of the .alpha.-alkenyl aromatic hydrocarbon to the reaction medium containing a vinyl ester at a temperature which enables the medium to be stirred so as to dissipate the heat of reaction. The gradual addition of the .alpha.-alkenyl aromatic monomer retards the overall polymerization rate so as to slow the evolution of heat and allows one to maintain a temperature of reaction conducive to heat removal by stirring.